The invention relates to a high pressure discharge lamp having a discharge vessel with opposing end chambers and a longitudinal axis, a discharge sustaining filling within said discharge vessel, and a pair of opposing discharge electrodes arranged in said end chambers and between which a discharge is maintained during lamp operation.
Such lamps are well known in the art and include, for example, mercury vapor and metal halide lamps. The discharge vessel for these lamps is typically of quartz glass (fused silica). Mercury lamps include a fill of mercury and a rare gas while metal halide lamps further include one or more metal halides which contribute to the emission spectrum of the lamp.
The fused silica discharge vessels of high wattage metal halide lamps (i.e. 250-400 W) are typically formed from a circular-cylindrical tube of this material and include press seals at each end formed by collapsing the tube ends with opposing press jaws. The discharge vessel has a center tubular portion, which retains the circular cross-section of the tube, and opposing end chambers of continuously reducing cross section which result from pressing of the press seals. The end chambers also include crevices at the region of the press seal which are formed as a result of the pressing process.
The conventional electrode which is in almost universal use for commercially available HID lamps includes a tungsten rod around which is wound a tungsten coil structure. The electrode rod protrudes from the coil and terminates at a tip to which the discharge arc attaches. The coil structure also facilitates starting and disposes of heat by thermal radiation. Various electrodes of this type are shown in the book High Pressure Discharge Lamps by W. Elenbaas, Philips Technical Library, Chapter 4, Section 2.2, pp. 116-117. The electrode rod is typically aligned with the longitudinal axis of the discharge vessel.
In high pressure discharge lamps, the photometric parameters (luminous efficacy, color rendering, color temperature) are dependent on the partial gas pressures of the fill constituents. The gas pressures, in turn, are primarily controlled by the surface temperature of the discharge vessel in the area at which the vapor condenses. In mercury and metal halide lamps the fill constituents are generally over dosed by a factor of about 100, i.e. during operation 1/100 th of the amount dosed is present in the vapor phase. The overdose fills up crevices in the area of the pinch and ensures that the salts condense at a location which is warmer than in the crevices. The condensing fill constituents coming out of the vapor phase, halide salts in the case of metal halide lamps, condense on the inner discharge vessel wall as liquid droplets of various sizes varying from a film like material to drops of about 1 mm. In a vertically operated metal halide lamp with conventional electrodes, the excess salts will condense in the lower end chamber at a location located axially between the electrode tip and the end of the end chamber. Some of the condensed liquid typically runs down the wall and pools in the crevices behind the electrode.
The temperature of the area where the fill condenses is influenced significantly by how close the discharge arc, determined by the inserted length of the electrode tip, is spaced from the end of the end chamber. The inserted length is determined by such factors as the number of coil turns necessary for holding a sufficient quantity of emitter material and/or for cooling the electrode rod, the spacing between the distal tip of the electrode rod and the adjacent coil turn to ensure that the arc attaches at the distal tip and not the coil, and the spacing of the rear-most coil turn from the wall of the end chamber to prevent spontaneous color burst caused by excess salts contacting the coil. Additionally, if the electrode is too short the heat conducted down the electrode rod from the discharge arc into the seal area will cause the fused silica to recrystallize and shorten lamp life through seal failure. These considerations place a practical limit on how much a conventional electrode can be shortened while still maintaining lamp integrity. The inserted length of the distal tip of a conventional electrode from the end chamber wall behind the electrode is typically on the order of several mm, for example 3-5 mm, for commercially available lamps.
The wall temperatures of the discharge vessel necessary to achieve acceptable lamp performance are generally not obtained in lamps with conventional electrodes solely through heating by the discharge arc. Rather, the wall temperature is increased according to one technique by providing the outside of the end chamber with a heat conserving coating of, for example, zirconium oxide. Such coatings are on almost all commercially available metal halide lamps. However, they are a disadvantage because they increase the cost of the lamp merely by their application. They are also the source of problems such as poor adhesion and discoloration, as well as being a source of spread in photometric results. Another technique is too provide a light-transmissive sleeve closely spaced about the discharge vessel, as is known from U.S. Pat. No. 4,888,517 (Keefe et al).
During the early 1980's, interest in low wattage (35-100 W) metal halide lamps rapidly developed, particularly as replacements for incandescent and halogen lamps for general interior and display lighting. The discharge vessels for early low wattage lamps were basically smaller versions of conventional high wattage discharge vessels and included conventional discharge electrodes of reduced size. The performance of these early low-wattage lamps was inferior to the efficacy (LPW) and color rendering (CRI) values in the region previously established for high wattage lamps, especially in the smaller 50 W and 70 W sizes. It was found that luminous efficacy and color rendering generally decreased as the size and wattage of the discharge vessel were reduced. Later efforts to improve the performance of low wattage lamps concentrated mainly on discharge vessel shaping and miniaturization of the end chambers and press seals. Formed-body discharge vessels, with precise elliptical or ovoidal bodies to provide a more nearly isothermal operation, resulted from these efforts. Such a discharge vessel is known, for example, from U.S. patent application Ser. No. 423,904 filed Oct. 19, 1989. These discharge vessels are of high quality, but are more expensive to manufacture than discharge vessels pressed from straight tubing. Shaping of the formed body requires repetitive, time consuming glass working steps which are not required for straight body discharge vessels.
On rare occasions, the discharge vessels of metal halide lamps may rupture with sufficient force to cause failure of the outer envelope. Accordingly, for lamps intended for open fixtures which do not have a separate cover to contain glass fragments (typically low wattage lamps), it is known to provide a containment sleeve around the discharge vessel to prevent failure of the outer envelope. Such a lamp is known from U.S. Pat. No. 5,136,204. The presence of the sleeve, however, complicates lamp construction because it must be supported about the discharge vessel. In the commercially available lamp according to this patent, the sleeve is quartz glass and has a wall thickness of 2 mm. Metal clips are secured on the press seals and include portions which hold the ends of the sleeve. The sleeve and discharge vessel are supported by welding the clips to an elongate metal support rod which is fixed around the lamp stem by a metal strap. The support rod, and consequently the metal clips and the sleeve, are electrically isolated which prevents accelerated sodium depletion from the discharge vessel. (For a detailed description of this sodium loss process, reference may be made to the textbook Electric Discharge Lamps by Dr. John Waymouth, M.I.T. Press 1971 (Chapter 10)). As compared to a typical nonshielded lamp in which the elongate support rod is welded to a stem conductor or is sealed in the stem to carry current to the discharge vessel, the fixing of the support rod to the stem with a metal strap is more expensive and intricate. The clips further add to the number of lamp parts and increase lamp cost.
From U.S. Pat. No. 4,721,876 it is known to surround a glass containment sleeve with a meshwork of metal wire which is fixed around the tube with metal clamping strips. The meshwork increases the containment capability of the sleeve. The sleeve is supported by clamping strips which are electrically conducting and connected to a lamp frame which supports the discharge vessel and connects the discharge electrodes to a source of electrical potential. The meshwork as a result is not electrically isolated, which can lead to the disappearance of sodium from the discharge vessel. The manufacture of the meshwork, or of a braided assembly, and its manipulation are also difficult.
FIG. 1 shows a discharge vessel/containment shield assembly 1 for a low wattage metal halide lamp which has been publicly disclosed by Venture Lighting Company. The discharge vessel is of the previously mentioned formed-body type in which the body portion 6, which lies between the press seals and in which the discharge is maintained, has a precise elliptical or ovoidal shape. One end 3 of the containment sleeve is open while the other end 4 is fused to both major faces of one of the press seals 5. The fusing of the sleeve to one of the discharge vessel press seals is advantageous because no additional metal parts are introduced into the lamp envelope. However, as compared to designs which use straps or clips to hold the sleeve, the containment provided by the construction of the lamp of FIG. 1 was found to be insufficient. The wall thickness of the sleeve in the lamp of FIG. 1 was 2 mm. In tests in which the discharge vessel was ruptured by a current surge, failure of the outer envelope was found to occur. Additionally, the sleeve construction is asymmetric in that the pinched end of the sleeve is totally closed whereas the other side is open. The lower end of the discharge vessel will thus have a significantly different temperature, and the lamp will have different photometrics, depending on whether the lamp orientation is base-up or base-down, which is undesirable.
While there are numerous combinations of metal halides which can be used in metal halide lamps, those which have enjoyed commercial success fall into two main categories. The thallium-indium-sodium combination offers excellent color rendering properties and has enjoyed commercial success in Europe. In the United States, the sodium scandium lamp has become practically universally accepted, due to its very good luminous efficacy, (typically 85 to 90 lumens per watt for lamps in the 250-400 W range) and long operating life (typically 10,000 to 15,000 hours).
Commercially available metal halide lamps universally include thorium either in the tungsten electrode, the fill material, or both. In the thallium-containing metal halide lamps, the electrodes carry an emitter material of thorium oxide retained in a reservoir formed by the turns of the tungsten coil structure. In operation, the thorium oxide is believed to decompose slightly and release free thorium to supply a monolayer film having reduced work function and higher emission. Unfortunately, this cathode cannot be used in a scandium-containing lamp because the ScI.sub.3 is converted to Sc.sub.2 O.sub.3, resulting in loss of essentially all the scandium in a relatively short time. Instead, a thoriated tungsten electrode is used in an iodide-containing atmosphere. Under proper conditions the thoriated rod serves as a good electron emitter. Additionally, it is also known to provide thorium iodide in the fill material, which supports a transport cycle which returns to the electrode any thorium lost to the discharge. The thorium-tungsten electrode and its method of operation are described in the book Electric Discharge Lamps by John F. Waymouth, M.I.T. Press, 1971, Chapter 9.
The use of thorium is a disadvantage in that it is a radioactive material, which creates serious problems during manufacture of thoriated tungsten electrodes and/or in dosing thorium iodide and thorium tetraiodide into the discharge vessel. For example, when thoriated tungsten electrodes are etched during their manufacture, thorium is dissolved in the etch liquid, making it a radioactive waste.
Without thorium, however, metal halide lamps with conventional electrodes have been found to experience rapid blackening of the discharge vessel. Electrodes without thorium have a high work function. The electrode must then run hotter to sustain the arc current, which causes an increase in tungsten evaporation from the electrode and unacceptable discharge vessel blackening within several hundred hours. This adversely effects lamp performance by reducing lumen maintenance and leads to reduced lamp life. The thorium layer is also critical because it protects the tungsten electrode from corrosion.
U.S. Pat. No. 3,937,996 (Cap) discloses a high wattage metal halide lamp which uses electrodes of refractory metal wire formed into large open loops and which do not include an electrode rod. The patent specifically teaches that the discharge vessel fill must include a metal compound of low work function which is pyrolitically decomposable and subject to plating out in the electrodes and which participates in a transport cycle which continuously returns the low work function metal to the electrode. In the disclosed embodiment, the discharge sustaining fill includes thorium iodide and thorium tetraiodide to maintain the thorium transport cycle discussed above.
In the early 1990's, the market calls for more cost effective low-wattage metal halide lamp designs which can safely be used in open fixtures. However, cost reduced lamps will only be commercially successful if the photometrics of the lamps are acceptable. For low-wattage metal halide lamps to be considered "standard quality", the efficacy should be greater than about 80 LPW for 100 watt lamps, greater than about 75 for 70 watt lamps and greater than about 65 for 50 watt lamps. The CRI should be greater than about 60.
It is the object of the invention to provide a high pressure discharge lamp, and particularly a low wattage metal halide lamp, which is of a simple, less costly and reliable construction while providing commercially acceptable photometrics.